gun(15) = 2 + gun(10) + gun(13)
now we find value of gun(10)
gun(10) = 2 + gun(5) + gun(8)
now we find value of gun(5)
gun(5) = 2 + gun(0) + gun(3)
Now gun(0)=1 and gun(3) =1
gun(5) = 2+1+1 = 4
similarly gun(8) = 2 + gun(3) + gun(6)
= 2 + 1 + 2 + gun(1) + gun(4) = 2+1+2+1+1 =7
so gun(10) = 2 + 4 + 7 = 13
Now find out gun(13)
gun(13) = 2 + gun(8) + gun(11)
we already know value of gun(8). so we will find value of gun(11)
gun(11) = 2 + gun(6) + gun(9)
= 2 + ( 2 + gun(1) + gun(4) ) + ( 2 + gun(4) + gun(7) )
= 2 + 2 + 1 + 1 + 2 + 1 + gun(7)
= 9 + (2 + gun(2) + gun(5) )
= 9 +2 + 1 + (2 + gun(0) + gun(3) )
= 12 + 2 + 1 + 1
so gun(11) = 16
so gun(13) = 2 + 7 + 16 = 25
hence gun(15) = 2 + 13 + 25 = 40
So answer should be 40